If I were inclined to say that black people were deliberately excluded from this movie (which I am not inclined to do, by the way)
That's the problem, though. A lot of exclusion isn't deliberate; it's just the unconscious perpetuation of inequalities built into the culture. The people in authority have been conditioned all their lives to default to whiteness and they don't even realize there's anything wrong with that. That's why it needs to be called attention to, why change needs to be actively pushed for -- because you can do something harmful or unfair without realizing it, and I'd like to think that most people would readily choose to stop doing it once they understood the problem. If they refuse to change things even after they've been made aware of the problem, if they willfully deny the problem or double down on the injustice, then it becomes deliberate and malicious.
Again, we are talking about a 40-year-old movie here. This problem was not uncommon at the time. It's still all too common in today's movie industry, but things are changing for the better, though much more so in TV than in features so far.